The present invention relates to a step-up power supplying circuit for stepping-up a DC voltage which is obtained by rectifying and smoothing an AC voltage and for supplying the step-up DC voltage to a load and, more particularly, to a step-up power supplying circuit for stepping-up a DC voltage by using a chopper circuit and for supplying the step-up DC voltage to a load.
As a conventional step-up power supplying apparatus using a chopper circuit, for instance, there is an apparatus as shown in FIG. 1.
In FIG. 1, reference numeral 10 denotes a commercially available AC power source; 12 and 14 input terminals to which a commercially available AC voltage is supplied from the commercially available AC power source 10; 16 a rectifying circuit which constructs a diode bridge by four diodes D.sub.1, D.sub.2, D.sub.3, and D.sub.4 and rectifies the AC input voltage; and 20 a capacitor for smoothing the voltage which has been rectified by the rectifying circuit 16. The rectifying circuit 16 and the capacitor 4 construct a rectifying and smoothing circuit 18 for converting the AC voltage into the DC voltage.
A chopper circuit 36 is subsequently provided. The chopper circuit 36 has a choke coil 22 and a transistor 24 serving as a switching device. The transistor 24 is turned on/off by a switching operation by high frequency pulses which are generated from an oscillator 26, thereby intermittently inducing an energy in the choke coil 22.
Reference numeral 28 denotes a diode for rectifying and 30 indicates a capacitor for smoothing. The intermittent energy induced in the choke coil 22 is rectified by the diode 28 and is smoothed by the capacitor 30 and is supplied to a load 34 through an output terminal 32.
When the commercially available AC voltage is supplied from the AC power source 10 to the input terminals 12 and 14, the commercially available AC voltage is rectified by the rectifying circuit 16 and is smoothed by the capacitor 20 and is supplied as a DC voltage to the chopper circuit 36.
The intermittent energy is induced in the choke coil 22 by the high frequency switching operation of the transistor 24 by the oscillator 26 in the chopper circuit 36. The intermittent energy induced in the choke coil 22 is rectified by the diode 28 and is smoothed by the capacitor 30 and is supplied to the load 34 through the output terminal 32 as a DC voltage which has been stepped up to a voltage level higher than the level of the input voltage from the rectifying and smoothing circuit 18 and has been regulated.
However, in such a conventional step-up power supplying circuit, there is a problem such that when the transistor 24 performs the switching operation, a high frequency ripple current flows from the choke coil 22 to the capacitor 20 to smooth the input voltage and a high frequency ripple voltage is developed in the capacitor 20, so that a high frequency noise is multiplexed between the input terminals 12 and 14.